Retroviral mediated gene-transfer of important and relevent genes such as v-abl (an oncogene) and IL3 (the growth factor for hematopoietic stem cells) into hemopoietic stem cells will provide information about regulation of hemopoietic stem cell behavior. Recently, we have established an in vitro system which allows growth of hemopoietic colonies consisting of up to 90% stem cells with self-renewal capacity. Infection of colonies with v-abl virus resulted in differentiation of these early stem cells to tumorigenic, immortalized mast cells which are growth factor independent. A number of recombinant IL3 and GM-CSF retrovirus have also been constructed. On infection of a factor-dependent cell line with these viruses, factor-independent growth was observed. Using cells for primary tissues such as fetal liver and adult mouse bone marrow, the results indicate that fetal liver contains a higher frequency of IL3 virus target stem cells. Our preliminary studies indicate that these infected stem cells can also reconstitute lethally irradiated recipients and express the viral IL3 gene.